The seismic risk to southern Ontario is a significant social and economic issue due to its density of population and concentrations of critical facilities, such as nuclear power plants. Recent moderate earthquakes, such as the magnitude 3.8 earthquake on Nov. 26, 1999 in Lake Ontario near Toronto, and the magnitude 5.2 earthquake on Jan. 1, 2000 near Temiskaming, Quebec, have contributed to enhanced public concern. A  recent swarm of felt earthquake activity near Madoc, Ontario, has also generated significant interest. Much of the concern stems from uncertainties in the occurrence rates and spatial distribution of earthquakes, as well as uncertainties in the severity of the ground shaking produced by earthquakes. Due to the relatively infrequent rates of earthquake occurrence in these areas, improved determination of these parameters requires either many decades of data gathering using sparsely distributed recording equipment, or several years of data gathering using a greater density of recording equipment. Some critical parameters, such as the depth distribution of earthquakes, can only be determined through a dense recording array. Improved understanding of the architecture of the Precambrian basement in southern Ontario is also required for a more complete picture of the earthquake hazardsin this region.

The Ontario POLARIS array will include 30 seismographs installed in the southern Ontario region, roughly from Ottawa to
Windsor and from Niagara to Georgian Bay (Figure), in conjunction with an MT study. This array will provide greatly
improved definition of the spatial and depth distribution of earthquakes and their occurrence rates, as well as crucial
information on generation and propagation of the seismic waves that cause damage to engineered structures. In addition, the
seismograph and MT arrays will furnish data for three-dimensional mapping of the lithospheric architecture, and for
identifying potential zones of crustal weakness.

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